U.S. patent application number 10/799845 was filed with the patent office on 2006-09-21 for nanoscale helical microstructures and channels from chiral poly(l-lactide) block containing block copolymers.
Invention is credited to Yi-Chun Chen, Yeo-Wan Chiang, Tsai-Ming Chung, Rong-Ming Ho, Bao-Tsan Ko, Chu-Chieh Lin, Hsi-Hsin Shih, Jassy S.J. Wang.
Application Number | 20060211816 10/799845 |
Document ID | / |
Family ID | 37011248 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060211816 |
Kind Code |
A1 |
Ho; Rong-Ming ; et
al. |
September 21, 2006 |
NANOSCALE HELICAL MICROSTRUCTURES AND CHANNELS FROM CHIRAL
POLY(L-LACTIDE) BLOCK CONTAINING BLOCK COPOLYMERS
Abstract
A method for making a series of nanoscale microstructures,
including helical microstructures and cylindrical microstructures.
This method includes the steps of: (1) forming a chiral block
copolymer containing a plurality of chiral first polymer blocks and
a second polymer blocks wherein the chiral first polymer blocks
have a volume fraction ranging from 20 to 49%; (2) causing a phase
separation in the chiral block copolymer. In a preferred
embodiment, the chiral block copolymer is
poly(styrene)-poly(L-lactide) (PS-PLLA) chiral block copolymer, and
the copolymerization process is a living copolymerization process
which includes the following steps: (a) mixing styrene with BPO and
4-OH-TEMPO to form 4-hydroxy-TEMPO-terminated polystyrene; and (2)
mixing the 4-hydroxy-TEMPO-terminated polystyrene with
[(.eta..sub.3-EDBP)Li.sub.2].sub.2[(.eta..sub.3-.sup.nBu)Li(0.5Et.sub.2O)-
].sub.2 and L-lactide in an organic solvent preferably
CH.sub.2Cl.sub.2 to form the poly(styrene)-poly(L-lactide) chiral
block copolymer. Transmission electron microscopy (TEM) and small
X-ray scattering (SAXS) studies show that when the volume fraction
of poly(L-lactide) is about 35-37%, nanoscale helices with a pitch
of 43.8 nanometers and a diameter of 34.4 nanometers were
observed.
Inventors: |
Ho; Rong-Ming; (Hsinchu,
TW) ; Chiang; Yeo-Wan; (Taichung, TW) ; Lin;
Chu-Chieh; (Taichung, TW) ; Ko; Bao-Tsan;
(Taichung, TW) ; Chen; Yi-Chun; (Hsinchu, TW)
; Chung; Tsai-Ming; (Hsinchu, TW) ; Shih;
Hsi-Hsin; (Hsinchu, TW) ; Wang; Jassy S.J.;
(Hsinchu, TW) |
Correspondence
Address: |
LAW OFFICE OF LIAUH & ASSOC.
4224 WAIALAE AVE
STE 5-388
HONOLULU
HI
96816
US
|
Family ID: |
37011248 |
Appl. No.: |
10/799845 |
Filed: |
March 12, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60454764 |
Mar 14, 2003 |
|
|
|
60467022 |
Apr 30, 2003 |
|
|
|
60472377 |
May 20, 2003 |
|
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|
Current U.S.
Class: |
525/55 ;
525/415 |
Current CPC
Class: |
G03F 7/0002 20130101;
C08L 53/00 20130101; C08F 293/005 20130101 |
Class at
Publication: |
525/055 ;
525/415 |
International
Class: |
C08F 8/00 20060101
C08F008/00 |
Claims
1. A method for making a series of nanoscale microstructures
comprising the steps of: (1) forming a chiral block copolymer
containing a firs-t polymer block of a first polymer and a second
polymer block of a second polymer, wherein at least said first
polymer is a chiral polymer exhibiting chirality, and said first
and second polymer blocks are capable of being subject to a
micro-phase separation and said first polymer has a volume fraction
ranging from 10 to 90%; (2) causing a microphase separation in said
chiral block copolymer; wherein said first polymer is
poly(L-lactide) and said second polymer is selected from the group
consisting of polystyrene and pol(4-vinylpyridine), further wherein
said chiral block copolymer is poly(styrene)-poly(L-lactide)
(PS-PLLA) chiral block copolymer when said second polymer is
polystyrene and poly(4-vinylpyridine)-poly(L-lactide) (P4VP-PLLA)
chiral block copolymer when said second polymer is
pol(4-vinylpyridine); further wherein said
poly(styrene)-poly(L-lactide) (PS-PLLA) chiral block copolymer is
prepared using a polymerization process comprising the following
steps: (a) mixing styrene with BPO and 4-OH-TEMPO to form
4-hydroxy-TEMPO-terminated polystyrene; and (b) mixing said
4-hydroxy-TEMPO-terminated polystyrene with L-lactide in an organic
solvent to form said poly(styrene)-poly(L-lactide) chiral block
copolymer.
2. The method for making a series of nanoscale microstructures
according to claim 1, wherein said chiral block copolymer is
poly(styrene)poly(L-lactide) (PS-PLLA) chiral block copolymer, said
first polymer is poly(L-lactide), and said second polymer is
polystyrene.
3. The method for making a series of nanoscale microstructures
according to claim 1, wherein said chiral block copolymer is
poly(4-vinylpyridine)-poly(L-lactide) (P4VP-PLLA) chiral block
copolymer, said first polymer is poly(L-lactide), and said second
polymer is pol(4-vinylpyridine).
4. The method for making a series of nanoscale microstructures
according to claim 1, wherein said first polymer has a volume
fraction ranging from about 20% to about 49%.
5. The method for making a series of nanoscale microstructures
according to claim 1, wherein said nanoscale microstructures are a
series of helical microstructures.
6. The method for making a series of nanoscale microstructures
according to claim 1, wherein said nanoscale microstructures are a
series of cylindrical microstructures each with a hexagonal
crossection.
7. (canceled)
8. The method for making a series of nanoscale microstructures
according to claim 1, wherein said polymerization process is a
living polymerization in which monomers are sequentially added to a
polymerization mixture.
9. The method for making a series of nanoscale microstructures
according to claim 1, wherein said phase separation of said chiral
block copolymer is achieved through crystallization.
10. A process for making an article of manufacture containing a
series of repeating nanoscale microstructures formed in a substrate
which is formed using a process comprising the steps of: (1)
forming a chiral block copolymer containing a first polymer block
of a first polymer and a second polymer block of a second polymer,
wherein at least said first polymer is a chiral polymer exhibiting
chirality, and said first and second polymer blocks are capable of
being subject to a micro-phase separation and said first polymer
has a volume fraction ranging from 10 to 90%; (2) causing a
microphase separation in said chiral block copolymer; wherein said
first polymer is poly(L-lactide) and said second polymer is
selected from the group consisting of polystyrene and
pol(4-vinylpyridine), further wherein said chiral block copolymer
is poly(styrene)-poly(L-lactide) (PS-PLLA) chiral block copolymer
when said second polymer is polystyrene and
poly(4-vinylpyridine)-poly(L-lactide) (P4VP-PLLA) chiral block
copolymer when said second polymer is pol(4-vinylpyridine); further
wherein said poly(styrene)-poly(L-lactide) (PS-PLLA) chiral block
copolymer is prepared using a polymerization process comprising the
following steps: (a) mixing styrene with BPO and 4-OH-TEMPO to form
4-hydroxy-TEMPO-terminated polystyrene; and (b) mixing said
4-hydroxy-TEMPO-terminated polystyrene with L-lactide in an organic
solvent to form said poly(styrene)-poly(L-lactide) chiral block
copolymer.
11. The process according to claim 10, wherein said block copolymer
is a poly(styrene)-poly(L-lactide) chiral block copolymer, and said
first polymer is poly(L-lactide) and said second polymer is
polystyrene.
12. The process according to claim 10 wherein said block copolymer
is a poly(4-vinylpyridine)-poly(L-lactide) chiral block copolymer,
and said first polymer is poly(L-lactide) and said second polymer
is poly(4-vinylpyridine).
13. The process according to claim 10 wherein said first polymer
has a volume fraction ranging from about 20% to about 49%.
14. The process according to claim 10 wherein said nanoscale
microstructures are a series of helical microstructures.
15. The process according to claim 10 wherein said nanoscale
microstructures are a series of cylindrical microstructures each
with a hexagonal crossection.
16. (canceled)
17. The process according to claim 10 wherein said polymerization
process is a living polymerization in which monomers are
sequentially added to a polymerization mixture.
18. The process according to claim 10 wherein said phase separation
of said chiral block copolymer is achieved through
crystallization.
19-21. (canceled)
Description
[0001] The present invention claims priority of provisional
applications Nos. 60/454,764, filed Mar. 14, 2003; 60/467,022,
filed, Apr. 30, 2003; and 60/472,377, filed May 20, 2003. The
contents therein are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a method for preparing
nanoscale helical microstructures and microchannels using chiral
block copolymers, and the series of systems and articles made
therefrom. More specifically, the present invention relates to a
method for preparing nanoscale helical microstructures and
microchannels using chiral block copolymers, and the series of
systems and articles made therefrom.
BACKGROUND OF THE INVENTION
[0003] PCT patent application No. WO 00/02090 describes an
elaborate procedure for preparing polymeric articles having a
three-dimensionally periodic structure of a plurality of
periodically occurring separate domains, with at least a first and
a second domain each being topologically continuous and with the
first domain comprising a polymeric species containing an inorganic
species capable of forming a ceramic oxide. It is desirable to
explore other approaches that could be more cost-effective.
[0004] Self-assembly of block copolymers have received extensive
attention because of their fascinating phase behavior. Some of the
studies have been reported by F. S. Bates in Science, 251, 898
(1991), and F. S. Bates and G. H. Fredrickson in Phys. Today, 32
(1990). Nanoscale microstructures from microphase separation can be
obtained simply by chemically joining immiscible polymer chains
together as block copolymers. Thermodynamically, the formed
microstructure is the lowest Gibb free energy of morphology by
balancing the enthalpic penalty and the entropic driving force of
mixing, expressing as .chi.N, the product of Flory-Huggins
segmental interaction parameter .chi. and the degree of
polymerization N. One-, two-, and three-dimensional periodicities
of self-assembled morphologies in bulk such as lamellar,
cylindrical and gyroid microstructures have been identified with
respect to various fractions of constituted blocks. These are
reported in, e.g., F. S. Bates in Science, 251, 898 (1991); F. S.
Bates and G. H. Fredrickson in Phys. Today, 32 (1990); S. Forster,
A. K. Khandpur, J. Zhao, F. S. Bates, I. W. Hamley, A. J. Ryan,
Bras W, Macromolecules 27, 6922 (1994); Vanessa Z.-H. Chan, J.
Hoffman, V. Y. Lee, H. Iatrou, A. Avgeropoulos, N. Hadjichristidis,
R. D. Miller, E. L. Thomas, Science 286, 1716 (1999); and F. S.
Bates and G. H. Fredrickson, Annu. Rev. Phys. Chem. 41, 525 (1990).
Defined textures can be easily tailored by molecular engineering of
synthetic block copolymers that appears promising in the
applications of nano-technologies. Examples of these reports can be
found in M. Park, C. K. Harrison, P. M. Chaikin, R. A. Register, D.
H. Adamson, Science 276, 1401 (1997); D. E. Fogg, L. H.
Radzilowski, R. Balnski, R. R. Schrock, E. L. Thomas,
Macromolecules 30, 417 (1997); Y. N. C. Chan, R. R. Schrock, R. E.
Cohen, Chem. Mater. 4, 24 (1992); S. Fo{umlaut over ( )}rster, M.
Antonietti, Adv. Mater. 10, 195 (1998); B. M. Discher, H. Bermudez,
D. A. Hammer, D. E. Discher, Y.- Y. Won, F. S. Bates, J. Phys.
Chem. B 106, 2848 (2002); A.-V. G. Ruzette, P. P. Soo, D. R.
Sadoway, A. M. Mayes, J. Electrochem. Soc. 148, A537 (2001); N.
Matsumi, K. Sugai, H. Ohno, Macromolecules 35, 5731 (2002); M. A.
Hillmyer, P. M. Lipic, D. A. Hajduk, K. Almdal, F. S. Bates, J. Am.
Chem. Soc. 119, 2749 (1997); P. M. Lipic, F. S. Bates, M. A.
Hillmyer, J. Am. Chem. Soc. 120, 8963 (1998); J. M. Dean, P. M.
Lipic, R. B. Grubbs, R. F. Cook, F. S. Bates, J. Polym. Sci., Part
B: Polym. Phys. 39, 2996 (2001); and M. R. Buchmeiser, Angew.
Chem., Int. Ed. 40, 3795 (2001).
[0005] Nature also employs the self-assembly of compounds as a tool
for structuring substances. Biological architectures are formed by
the interplay among steric, hydrophobic, hydrogen-bonding,
electrostatic interactions to form different levels of
organization, i.e., different length-scales of morphologies. These
morphologies are built from the smallest building blocks, e.g.,
amino acids, to achieve higher levels of organization. The
self-assembly of synthetic supramolecules since its early days has
been inspired by using these secondary interactions of biological
matters, and has already created a large number of nanoscale
architectures. Pursuing biological-like architectures by using
synthetic systems has been, and still is the essential goals of
many researchers in the fields of chemistry and physics. Among
them, helical morphologies of different length-scales varying from
helical chain conformations, helical aggregations to helical
agglomerates are the most fundamental and interesting textures in
biological systems. Discussions on this subject can be found in,
e.g., H. Engelkamp, S. Middelbeek, R. J. M. Nolte, Science 284, 785
(1999); A. R. A. Palmans, J. A. J. M. Vekemans, E. E. Havinga, E.
W. Meijer, Angew. Chem., Int. Ed. Engl. 36, 2648 (1997); T.
Tachibana, H. Kambara, J. Am. Chem. Soc. 87, 3015 (1965); R. J. H.
Hafkamp, B. P. A. Kokke, I. M. Danke, H. P. M. Geurts, A. E. Rowan,
M. C. Feiters, R. J. M. Nolte, Chem. Commun., 545 (1997); K.
Hanabusa, M. Yamada, M. Kimura, H. Shirai, Angew. Chem., Int. Ed.
Engl. 35, 1949 (1996); M. De Loos, J. van Esch, I. Stokroos, R. M.
Kellog, B. M. Feringa, J. Am. Chem. Soc. 119, 12675(1997); and J.
H. Van Esch, B. L. Feringa, Angew. Chem., Int. Ed. 39, 2263 (2000).
Y. Okamoto,; K. Suzuki, K. Ohta, K. Hatada, H. Yuki, J. Am. Chem.
Soc. 101, 4763 (1979) discussed the dissymmetric shapes of helical
textures result in specific chemical and physical characters such
as molecular recognition and optically active properties. S. I.
Stupp, V. LeBonheur, K. Walker, L. S. Li, K. E. Huggins, M. Keser,
and A. Amstutz, Science 276, 384(1997) discussed desirable reasons
for producing helical materials have been identified in the fields
of materials and life sciences.
[0006] The chirality of compounds has been referred to as one of
the main origins for the formation of helical textures, and used as
secondary interaction to assemble chiral molecules and
macromolecules into larger helical structures. These have been
discussed in, e.g., A. E. Rowan, R. J. M. Nolte, Angew. Chem., Int.
Ed., 37, 63 (1998); and M. C. Feiters, R. J. M. Nolte, in Advances
in Supramolecular Chemistry. Vol. 6. Chiral Self-assembled
Structures of Biomolecules and Synthetic Analogues; G. W. Gokel,
Ed.; JAI Press Inc.: Stamford, Conn.; Vol. 6, pp 41-156. Helical
superstructures with a specific handedness have been obtained from
buffer solutions of amphiphilic block copolymers containing charged
helical blocks. As described by Nolte and co-workers, the chiral
entity of constituted block aside from solvent, amphiphilicity and
electrostatic effects plays important role for the formation of
helical superstructures in solution. See, e.g., 29. J. J. L. M.
Cornelissen, M. Fischer, N. A. J. M. Sommerdijk, R. J. M. Nolte,
Science 280, 1427(1998); N. A. J. M. Sommerdijk, S. J. Holder, R.
C. Hiorns, R. G. Jones, R. J. M. Nolte, Macromolecules 33,
8289(2000). Hillmyer and co-workers studied
poly(styrene)-poly(D,L-lactide) (PS-PLA) and no helical
morphologies were found. See, A. S. Zalusky, R. Olayo-Valles, C. J.
Taylor, M. A. Hillmyer, J. Am. Chem. Soc. 123, 1519(2001); and A.
S. Zalusky, R. Olayo-Valles, J. H. Wolf, M. A. Hillmyer, J. Am.
Chem. Soc. 124, 12761(2002). It is well known that aliphatic
polyesters such as poly(caprolactone) (PCL) and poly(lactide) (PLA)
can be hydrolytically degraded owing to the unstable character of
ester group. See, J. L. Kathleen, M. Edith, Biomaterials 19,
1973(1998); A. Gopferich, Biomaterials 17, 103(1996); and W. K.
Lee, J. A. Gardella, Langmuir 16, 3401(2000).
SUMMARY OF THE INVENTION
[0007] The primary object of the present invention is to develop a
method for preparing nanoscale helical microstructures and
microchannels. More specifically, the primary object of the present
invention is to develop a method that uses chiral block copolymers
for preparing nanoscale helical microstructures and
microchannels.
[0008] In the present invention, unexpected results were observed
when poly(styrene)-poly(L-lactide) (PS-PLLA), chiral block
copolymers containing left-handed helical backbone of PLLA blocks,
were prepared by sequential living free radical and living
ring-opening polymerizations. "Living" polymeriztion for the
synthesis of block copolymers means the sequential addition of
monomers. Left-handed PLLA helices having nanoscale dimensions of
pitch length and diameter were formed from self-assembly PS-PLLA in
bulk. These PLLA helices are shaped as hexagonally packed
microstructure in PS matrix. In contrast to PS-PLLA, helical
textures have not been found in poly(styrene)-poly(D,L-lactide).
The formation of nanoscale, helical morphology driven by the
chirality of synthetic block copolymers creates new scopes for
block copolymer thermodynamics and applications. In a sense, the
method of the present invention mimics the biologically occurring
process in nature, in that the contribution of chirality aside from
usual secondary interactions plays important role in the
self-assembly of synthetic supramolecules. One of the main
advantages of the method of the present invention is that nanoscale
channels can be obtained by the hydrolysis of the PLLA blocks. The
present invention opens new opportunities for block copolymer
applications in the fields of nano-sciences.
[0009] In the present invention, diblock copolymers constituting
only achiral and chiral blocks, poly(styrene)-poly(L-lactide)
(PS-PLLA), were synthesized in this study to examine the chirality
effect on self-assembly morphologies. It was unexpected discovered
that well organized, hexagonally packed PLLA nanoscale helices with
a left-handed helical sense equal to that of the constituted block
were formed from PS-PLLA self-assembly in bulk. To the best of our
knowledge, no chiral block copolymer has yet been described that
generates chiral superstructures in bulk and self assembles to
three-dimensionally packed microstructures. The formation of
nanoscale, helical microstructures is attributed to the chiral
effect of block copolymers in addition to the van der Waals
interaction and the entropy of mixing. As discussed above, one of
the main advantages of the method of the present invention is that
nanoscale channels can be obtained by the hydrolysis of the PLLA
blocks.
[0010] A series of poly(styrene)-block-poly(L-lactide) (PS-PLLA)
copolymers with different volume ratio were prepared by using a
two-step polymerization sequence. Free radical polymerization of
styrene using 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl,
4-Hydroxy-TEMPO (4-OH-TEMPO) as initiator in the presence of
dibenzoylperoxide (BPO) resulted in a hydroxy-terminated
polystyrene which further reacts with
[(.mu..sub.3-EDBP)Li.sub.2].sub.2[(.mu..sub.3-.sup.nBu)Li(0.5Et.sub.2O)].-
sub.2 to form a macroinitiator. Followed by the control
ring-opening polymerization of L-lactide in the presence of the
macroinitiator, PS-PLLA was then prepared. Using this procedure,
PS-PLLA block copolymers having different amounts of PLLA were
obtained.
[0011] The self-assembly morphologies of various block copolymers
were studied using transmission electron microscopy (TEM) and
small-angle X-ray scattering (SAXS). Bulk samples of block
copolymers were prepared by solution-casting from dichloroethane
(CH.sub.2Cl.sub.2) solution (0.5 wt % of PS-PLLA) at room
temperature. It has been identified that crystallization effect is
significant on the morphology of crystallizable block copolymers.
To eliminate the disturbance of PLLA crystallization on the
so-formed morphology, cast samples were annealed at temperature
above the PLLA melting point but below its order-disorder
transition temperature, and then quenched in liquid nitrogen. The
measurements of PS-PLLA thermal behavior were carried out by
differential scanning calorimetry (DSC). The order-disorder
transition temperature of PS-PLLA was above degradation temperature
(.about.170.degree. C.) as identified by small-angle X-ray
scattering (SAXS). Amorphous PS-PLLA was obtained after the thermal
treatments as evidenced by differential scanning calorimetry (DSC)
and wide-angle X-ray measurements (WAXD).
[0012] After quenching from the microphase-separated melt, the
thermally treated samples were then sectioned by ultra-microtome
after RuO.sub.4 staining. A variety of microstructures such as
micelle, hexagonal cylinder, lamellae textures was identified by
TEM and SAXS when f.sub.PLLA.sup.v=0.17, 0.30, and 0.56,
respectively. A helical microstructure was observed when
f.sub.PLLA.sup.v=0.37 as shown in the TEM micrographs. The pitch
and the diameter of the nanoscale helices were determined as 43.8
nm and 34.4 nm in average, respectively. The ratio of pitch to
radius of the nanoscale helices (P/r=2.5) follows the general rule
of helical formation in Nature of which the P/r value must be below
6.28. Well-oriented microdomains consisting of hexagonally packed
PLLA twists in PS matrix were observed for cast samples by using
TEM. The occupied volume fraction of PLLA blocks was calculated as
40 vol %, this is approximately consistent with estimated value
from synthetic results (f.sub.PLLA.sup.v=0.37). Corresponding SAXS
experiments were performed to further identify the observed
morphology. Well-defined scattering pattern having six-fold
symmetry is evident in the two-dimensional (2D) SAXS results along
the helical axis whereas two-fold symmetry is obtained along the
axis normal of helices. Azimuthal integrated one-dimensional SAXS
profile was further obtained from the six-fold symmetry pattern.
The diffraction peaks occur at relative q* values of
1:4.sup.1/2:7.sup.1/2:9.sup.1/2 and a broad diffraction peak
appears at ca. 3.5q*. These results are consistent with the
hexagonally packed microstructures of which the nanoscale helices
can be treated as cylindrical columns for packing due to the
specific P/r value.
[0013] On the assumption of cylindrical packing, the 3.5q*
diffraction can be identified as the form factor contribution of
cylinder-like morphology in accordance with projected diameter
along the helical axis observed by TEM. Furthermore, the determined
long period, 50.6 nm, by SAXS is consistent with observed spacing
by TEM. As a result, the 3D geometrical arrangement of nanoscale
helical microstructure in bulk for the self-assembly of PS-PLLA was
clearly identified. Also, the helical morphology adopts a
left-handed helical sense equal to that of the constituted block as
observed by TEM. By contrast, the formation of nanoscale helical
microstructures has not been found in PS-PLA block copolymers and
PLLA homopolymer. Consequently, the appearance of the helical
microstructure for PS-PLLA is attributed to the effect of chirality
interacting with the immiscibility of constituted blocks and the
entropy of mixing.
[0014] In addition to PS-PLLA, block copolymers of
poly(4-vinylpyridine)-poly(L-lactide) (P4VP-PLLA) were also
synthesized using a process similar to those discussed in previous
examples. In a preferred embodiment, the volume fraction
poly(L-lactide) in the poly(4-vinylpyridine)-poly(L-lactide) block
copolymer was calculated to be about 23% (f.sub.PLLA.sup.v=0.23).
As shown in the TEM micrograph in a helical microstructure was also
observed.
[0015] The analyses of block copolymer thermodynamics are
complicated by the chiral entities of constituted components. This
will definitely be an impact to the conventional treatments of
block copolymers in theory. The helical behavior is expected by the
chiral sense of block copolymers. For applications, the hexagonally
packed helices were treated by hydrolysis of PLLA. As observed by
AFM, hexagonally packed nanohole arrays were generated on the
surface of solution-casting PS-PLLA samples after hydrolysis. To
further identify the texture of the hydrolyzed PS-PLLA samples,
field emission scanning electron microscopy was used to observe the
samples from different views. To view parallel to the helical axes,
ordered nanohole arrays were observed by FESEM and the results are
similar to the observations by SPM. To view perpendicular to the
helical axes, traces of degradation for PLLA helical morphology can
be identified. As a result, helical channels can be obtained by
simply hydrolyzing the PLLA helices.
BRIEF DESCRIPTION OF THE DRAWING
[0016] The present invention will be described in detail with
reference to the drawing showing the preferred embodiment of the
present invention, wherein:
[0017] FIG. 1A is a TEM micrograph of PS38-PLLA10
(f.sub.PLLA.sup.v=0.17)
[0018] FIG. 1B is a TEM micrograph of PS38-PLLA21
(f.sub.PLLA.sup.v=0.30)
[0019] FIG. 1C is a TEM micrograph of PS13-PLLA22
(f.sub.PLLA.sup.v=0.56)
[0020] FIG. 1D is a schematic drawing illustrating the nanoscale
micelle microstructure of PS38-PLLA 10 (f.sub.PLLA.sup.v=0.17)
[0021] FIG. 1E is a schematic drawing illustrating the nanoscale
hexagonal microstructure of PS38-PLLA21 (f.sub.PLLA.sup.v=0.30)
[0022] FIG. 1F is a schematic drawing illustrating the nanoscale
lamellae microstructure of PS38-PLLA22 (f.sub.PLLA.sup.v=0.56)
[0023] FIG. 2A is a TEM micrograph of PS29-PLLA22
(f.sub.PLLA.sup.v=0.37)
[0024] FIG. 2B is a schematic drawing illustrating the nanoscale
helical microstructure of PS29-PLLA22 (f.sub.PLLA.sup.v=0.37)
[0025] FIG. 3A is a three-dimensional TEM micrograph of a
PS29-PLLA22 (f.sub.PLLA.sup.v=0.37) solution-cast sample sectioned
along xyz planes; as shown, the xy planes are the basal plane of
the cast substrate and the yz and zx planes are vertical planes
with respect to the cast surface.
[0026] FIG. 3B shows two-dimensional SAXS diffraction patterns of
the PS29-PLLA22 (f.sub.PLLA.sup.v=0.37) cast sample along various
directions.
[0027] FIG. 4 is a plot showing the azimuthal integrated
one-dimensional SAXS profile of the PS29-PLLA22
(f.sub.PLLA.sup.v=0.37) solution-cast sample.
[0028] FIG. 5A is an SPM micrograph of the PS29-PLLA22
(f.sub.PLLA.sup.v=0.37) solution-cast sample before hydrolysis.
[0029] FIG. 5B is an SPM micrograph of the PS29-PLLA22
(f.sub.PLLA.sup.v=0.37) solution-cast sample after hydrolysis.
[0030] FIG. 6A is an FESEM micrograph of a hydrolyzed PS29-PLLA2
(f.sub.PLLA.sup.v=0.37) sample viewed parallel to the helical
axes.
[0031] FIG. 6B is an FESEM micrograph of a hydrolyzed PS29-PLLA2
(f.sub.PLLA.sup.v=0.37) sample viewed perpendicular to the helical
axes.
[0032] FIG. 7 is a flowchart diagram showing the process of
synthesizing the poly(styrene)-poly(L-lactide) (PS-PLLA) chiral
block copolymers of the present invention.
[0033] FIG. 8A shows one-dimensional SAXS profiles of PS280-PLLA127
(f.sub.PLLA.sup.v=0.35) sample extracted from 2D patterns by
azimuthal integration; diffraction peaks were found to occur at q*
ratios of 1:4.sup.1/2:7.sup.1/2:9.sup.1/2:13.sup.1/2.
[0034] FIG. 8B shows the reflections in the 2D SAXS diffraction
pattern of PS280-PLLA127 (f.sub.PLLA.sup.v=0.35) sample along the
direction of helical axes indicate that the reflections possess
almost identical spacing; the intensity of these reflections
exhibits a 2-fold symmetry.
[0035] FIG. 9 is a TEM micrograph of block copolymers of
Poly(4-vinylpyridine)-poly(L-lactide) (f.sub.PLLA.sup.v=0.37),
showing a helical microstructure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0036] The present invention will now be described more
specifically with reference to the following examples. It is to be
noted that the following descriptions of examples, including the
preferred embodiment of this invention, are presented herein for
purposes of illustration and description, and are not intended to
be exhaustive or to limit the invention to the precise form
disclosed.
EXAMPLE 1
Synthesis of 4-hydroxy-TEMPO-terminated Polystyrene (PS-2)
[0037] A mixture of styrene (46 mL, 400 mmol), BPO (0.39 g, 1.6
mmol) and 4-OH-TEMPO (0.33 g, 1.92 mmol) (molar ratio of
4-OH-TEMPO/BPO=1.2) was preheated in a round-bottom flask (250 mL)
in nitrogen atmosphere at 95.degree. C. for 3 h to allow BPO to
decompose completely. The system was then heated at 130.degree. C.
for another 4 h to yield PS-TEMPO-4-OH. The resulting polystyrene
was precipitated with methanol (300 mL) from a THF (50 mL)
solution.
[0038] The product was then recrystallized twice from
CH.sub.2Cl.sub.2 (40 mL)/MeOH (200 mL) mixed solution, and
collected by vacuum filtration to give white solids. The final
solid was washed by 100 mL MeOH and dried in vacuo overnight to
form PS-2 [yield: 32.6 g (78%). Mn=20900 and PDI=1.17. .sup.1H NMR
(CDCl.sub.3): 6.46-7.09 (br, 5H, ArH), 1.84 (br, 1H, CH), 1.42 (br,
2H, CH.sub.2)]. All manipulations were carried out under a dry
nitrogen atmosphere. Solvents, benzoyl peroxide, styrene,
L-lactide, and deuterated solvents were purified before uses.
EXAMPLE 2
Synthesis of Block Copolymers of Polystyrene-Poly(L-lactide)
(PS-b-PLA or CP-4)
[0039] A typical ring-opening polymerization procedure was
exemplified by the synthesis of CP-4.
[(.eta..sub.3-EDBP)Li.sub.2].sub.2[(.eta..sub.3-.sup.nBu)Li(0.5Et.sub.2O)-
].sub.2 (0.11 g, 0.1 mmol) was added to 4-hydroxy-TEMPO-polystyrene
(PS-2, 4.18 g, 0.2 mmol) in 20 mL of toluene at 0.degree. C. The
mixture was stirred at room temperature for 2 h, and then dried
under vacuum. The resulting product (lithium alkoxide
macroinitiator) was dissolved in CH.sub.2Cl.sub.2 (20 mL) and
L-lactide (2.16 g, 15 mmol) in CH.sub.2Cl.sub.2 (10 mL) was added.
While the mixture was stirred for 4 h, conversion yield (74%) of
poly(L-lactide) was analyzed by .sup.1H NMR spectroscopic
studies.
[0040] The mixture was then quenched by the addition of an aqueous
acetic acid solution (0.35 N, 20 mL) and the polymer was
precipitated on pouring into n-hexane (300 mL) to give white
solids. The product was purified by precipitation from
CH.sub.2Cl.sub.2 (30 mL)/Hexane (150 mL) mixture solution. The
final crystalline solid was precipitated from CH.sub.2Cl.sub.2 (30
mL)/MeOH (150 mL) and dried under vacuum at 50-60.degree. C.
overnight to yield 3.02 g of PS-b-PLA(CP-4) (yield: 48%). Mn=46700
and PDI=1.17. .sup.1H NMR (CDCl.sub.3): 6.46-7.09 (br, 5H, ArH),
5.16 (q, 1H, CH(CH.sub.3), J=7.2 Hz), 1.84 (br, 1H, CH), 1.58 (d,
3H, CH(CH.sub.3), J=7.2 Hz), 1.42 (br, 2H, CH.sub.2). .sup.1H and
.sup.13C NMR spectra were recorded on a Varian VXR-300 (300 MHz for
.sup.1H and 75 MHz for .sup.13C) or a Varian Gemini-200 (200 MHz
for .sup.1H and 50 MHz for .sup.13C) spectrometer with chemical
shifts given in ppm from the internal TMS or the central line of
CHCl.sub.3. The GPC measurements were performed on a Hitachi L-7100
system equipped with a differential Bischoff 8120 RI detector using
THF (HPLC grade) as an eluent. Molecular weight and molecular
weight distributions were calculated using polystyrene as
standard.
[0041] A number of poly(styrene)-poly(L-lactide) (PS-PLLA) chiral
block copolymers were prepared. On the basis of molecular weight
and volume ratio, these PS-PLLAs are designated as PSxx-PLLAyy
(f.sub.PLLA.sup.v=z), wherein xx and yy represent the molecular
weight of PS and PLLA divided by one thousand measured by NMR,
respectively, and z indicates the volume fraction of PLLA. In these
calculations, the densities of PS and PLLA are assumed to be 1.02
and 1.18 g/cm.sup.3, respectively.
EXAMPLE 3
Transmission Electron Microscopy (TEM) and Small-Angle X-ray
Scattering (SAXS Studies
[0042] Crystallization of PLLA in PS-PLLA gave rise to significant
changes for microphase-separated morphology of PS-PLLA as observed
in our laboratory. It is possible to destroy the formed
microstructures so as to form crystalline morphology. DSC
experiments were carried out in a Perkin Elmer DSC 7. For instance,
PLLA blocks of PS29-PLLA22 (f.sub.PLLA.sup.v=0.37) melt at around
165.degree. C. The maximum crystallization rate of PLLA blocks is
at ca. 95.degree. C. in accordance with exothermic response (i.e.,
the occurrence of crystallization) at different isothermal
crystallizations. However, no significant exothermic response was
observed under fast cooling. The glass transition temperatures of
PLLA and PS are approximately 51.4.degree. C. and 99.2.degree. C.,
respectively.
[0043] SAXS experiments were conducted at the synchrotron X-ray
beam-line X3A2 at the National Synchrotron Light Source in
Brookhaven National Laboratory. The wavelength of the X-ray beam is
0.154 nm. The zero pixel of the SAXS patter was calibrated using
silver behenate, with the first-order scattering vector q*
(q*=4.lamda..sup.-1 sin.theta., where 2.theta. is the scattering
angle) being 1.076 nm.sup.-1. Time-resolved SAXS experiments were
carried out in a heating chamber with step temperature increasing.
Degradation temperature was identified by the disappearance of
scattering peaks.
[0044] DSC thermogram appeared no melting endotherm during heating.
WAXD (Widel-Angle X-ray Diffraction) diffraction exhibited
amorphous diffraction profile. A Siemens D5000 1.2 kW tube X-ray
generator (Cu K.sub..alpha. radiation) with a diffractometer was
used for WAXD powder experiments. The scanning 20 angle ranged
between 5.degree. and 40.degree. with a step scanning of
0.05.degree. for 3 sec. The diffraction peak positions and widths
observed from WAXD experiments were carefully calibrated with
silicon crystals with known crystal size.
[0045] Transmission electron microscopy in bright field was
performed with JEOL TEM-1200.times. transmission electron
microscopy. Staining was accomplished by exposing the samples to
the vapor of a 4% aqueous RuO.sub.4 solution for 3 hours.
[0046] The surface of the solution-casting PS-PLLA samples after
hydrolysis was observed using AFM (Atomic Force Microscopy). A
Seiko SPA-400 AFM with a SEIKO SPI-3800N probe station was employed
at room temperature in this work. A rectangle-shaped silicon tip
was applied in dynamic force mode (DFM) experiments using a type of
SI-DF20 with a spring force contact of 19 Nm.sup.-1 and scan rate
of 1.0 Hz.
[0047] Field emission scanning electron microscopy was used to
observe the PS-PLLA samples from different views. Field emission
scanning electron microscopy (FESEM) was performed on a Hitachi
S-900 FE-SEM using accelerating voltages of 2-5 keV. Samples were
examined either on the solution-cast surface or fractured cross
sections of PS-PLLA thin films after hydrolysis. The samples were
mounted to brass shims using carbon adhesive, and then
sputter-coated with 2-3 nm of gold (the gold coating thickness is
estimated from a calculated deposition rate and experimental
deposition time).
EXAMPLE 4
Results of TEM and SAXS Studies
[0048] FIG. 1A is a TEM micrograph of PS38-PLLA10
(f.sub.PLLA.sup.v=0.17); FIG. 1B is a TEM micrograph of PS38-PLLA21
(f.sub.PLLA.sup.v=0.30); and FIG. 1C is a TEM micrograph of
PS13-PLLA22 (f.sub.PLLA.sup.v=0.56). FIG. 1D is a schematic drawing
illustrating the nanoscale micelle microstructure of PS38-PLLA10
(f.sub.PLLA.sup.v=0.17); FIG. 1E is a schematic drawing
illustrating the nanoscale hexagonal microstructure of PS38-PLLA21
(f.sub.PLLA.sup.v=0.30); and FIG. 1F is a schematic drawing
illustrating the nanoscale lamellae microstructure of PS38-PLLA22
(f.sub.PLLA.sup.v=0.56).
[0049] The self-assembly morphologies of various block copolymers
were studied using transmission electron microscopy (TEM) and
small-angle X-ray scattering (SAXS). Bulk samples of block
copolymers were prepared by solution-casting from dichloroethane
(CH.sub.2Cl.sub.2) solution (0.5 wt % of PS-PLLA) at room
temperature. It has been identified that crystallization effect is
significant on the morphology of crystallizable block copolymers.
To eliminate the disturbance of PLLA crystallization on the
so-formed morphology, cast samples were annealed at temperature
above the PLLA melting point but below its order-disorder
transition temperature, and then quenched in liquid nitrogen. The
measurements of PS-PLLA thermal behavior were carried out by
differential scanning calorimetry (DSC). The order-disorder
transition temperature of PS-PLLA was above degradation temperature
(.about.170.degree. C.) as identified by small-angle X-ray
scattering (SAXS). Amorphous PS-PLLA was obtained after the thermal
treatments as evidenced by differential scanning calorimetry (DSC)
and wide-angle X-ray measurements (WAXD).
[0050] After quenching from the microphase-separated melt, the
thermally treated samples were then sectioned by ultra-microtome
after RuO.sub.4 staining. A variety of microstructures such as
micelle, hexagonal cylinder, lamellae textures was identified by
TEM and SAXS when f.sub.PLLA.sup.v=0.17, 0.30, and 0.56,
respectively. These micelle, hexagonal cylinder, and lamellae
microstructures are shown in FIGS. 1D through 1F, respectively.
[0051] FIG. 2A is a TEM micrograph of PS29-PLLA22
(f.sub.PLLA.sup.v=0.37); and FIG. 2B is a schematic drawing
illustrating the nanoscale helical microstructure of PS29-PLLA22
(f.sub.PLLA.sup.v=0.37).
[0052] It was unexpected that a helical microstructure was observed
when f.sub.PLLA.sup.v=0.37 as shown in the TEM micrographs provided
in FIGS. 2A and 2B. The pitch and the diameter of the nanoscale
helices were determined as 43.8 nm and 34.4 nm in average,
respectively. The ratio of pitch to radius of the nanoscale helices
(P/r=2.5) follows the general rule of helical formation in Nature
of which the P/r value must be below 6.28.
[0053] FIG. 3A is a three-dimensional TEM micrograph of a
PS29-PLLA22 (f.sub.PLLA.sup.v=0.37) solution-cast sample sectioned
along xyz planes; as shown, the xy planes are the basal plane of
the cast substrate and the yz and zx planes are vertical planes
with respect to the cast surface. And FIG. 3B shows two-dimensional
SAXS diffraction patterns of the PS29-PLLA22
(f.sub.PLLA.sup.v=0.37) cast sample along various directions. As
shown in FIGS. 3A and 3B, well-oriented microdomains consisting of
hexagonally packed PLLA twists in PS matrix were observed for cast
samples by using TEM. The occupied volume fraction of PLLA blocks
was calculated as 40 vol %, this is approximately consistent with
estimated value from synthetic results (f.sub.PLLA.sup.v=0.37).
[0054] Corresponding SAXS experiments were performed to further
identify the observed morphology. Well-defined scattering pattern
having six-fold symmetry is evident in the two-dimensional (2D)
SAXS results along the helical axis whereas two-fold symmetry is
obtained along the axis normal of helices. Azimuthal integrated
one-dimensional SAXS profile was further obtained from the six-fold
symmetry pattern as shown in FIG. 4, which is a plot showing the
azimuthal integrated one-dimensional SAXS profile of the
PS29-PLLA22 (f.sub.PLLA.sup.v=0.37) solution-cast sample. The
diffraction peaks occur at relative q* values of
1:4.sup.1/2:7.sup.1/2:9.sup.1/2 and a broad diffraction peak
appears at ca. 3.5q*. These results are consistent with the
hexagonally packed microstructures of which the nanoscale helices
can be treated as cylindrical columns for packing due to the
specific P/r value.
[0055] As discussed above, based on the assumption of cylindrical
packing, the 3.5q* diffraction can be identified as the form factor
contribution of cylinder-like morphology in accordance with
projected diameter along the helical axis observed by TEM.
Furthermore, the determined long period, 50.6 nm, by SAXS is
consistent with observed spacing by TEM. As a result, the 3D
geometrical arrangement of nanoscale helical microstructure in bulk
for the self-assembly of PS-PLLA was clearly identified. Also, the
helical morphology adopts a left-handed helical sense equal to that
of the constituted block as observed by TEM. By contrast, the
formation of nanoscale helical microstructures has not been found
in PS-PLA block copolymers and PLLA homopolymer. Consequently, the
appearance of the helical microstructure for PS-PLLA is attributed
to the effect of chirality interacting with the immiscibility of
constituted blocks and the entropy of mixing.
[0056] The analyses of block copolymer thermodynamics are
complicated by the chiral entities of constituted components. This
will definitely be an impact to the conventional treatments of
block copolymers in theory. The helical behavior is expected by the
chiral sense of block copolymers.
[0057] FIG. 5A is an SPM (Scanning probe microscopy) micrograph of
the PS29-PLLA22 (f.sub.PLLA.sup.v=0.37) solution-cast sample before
hydrolysis. And FIG. 5B is an SPM micrograph of the PS29-PLLA22
(f.sub.PLLA.sup.v=0.37) solution-cast sample after hydrolysis. The
hexagonally packed helices were treated by subjecting the PLLA to
hydrolysis. As observed by AFM, hexagonally packed nanohole arrays
were generated on the surface of solution-casting PS-PLLA samples
after hydrolysis.
[0058] FIG. 6A is an FESEM micrograph of a hydrolyzed PS29-PLLA22
(f.sub.PLLA.sup.v=0.37) sample viewed parallel to the helical axes.
And FIG. 6B is an FESEM micrograph of a hydrolyzed PS29-PLLA22
(f.sub.PLLA.sup.v=0.37) sample viewed perpendicular to the helical
axes. To further identify the texture of the hydrolyzed PS-PLLA
samples, field emission scanning electron microscopy was used to
observe the samples from different views. To view parallel to the
helical axes, ordered nanohole arrays were observed by FESEM and
the results are similar to the observations by SPM. To view
perpendicular to the helical axes, traces of degradation for PLLA
helical morphology can be identified. As a result, helical channels
can be obtained by simply hydrolyzing the PLLA helices.
EXAMPLE 7
Flowchart Diagram
[0059] FIG. 7 is a flowchart diagram showing the process of
synthesizing the poly(styrene)-poly(L-lactide) (PS-PLLA) chiral
block copolymers of the present invention. Those steps described in
the flowchart diagram of FIG. 7 have been discussed in Example
2.
EXAMPLE 8
Synthesis of Block Copolymers of PS280PLLA127
[0060] Another series of PS-PLLA copolymers with different volume
ratios were prepared by the same two-step "living" polymerization
sequence. On the basis of molecular weight and volume ratio, these
PS-PLLAs were designated as PSx-PLLAy (f.sub.PLLA.sup.v=z), with x
and y representing the numbers of the repeating unit for PS and
PLLA blocks and z representing the volume fraction of PLLA
(calculated by assuming that densities of PS and PLLA were 1.02 and
1.18 g/cm.sup.3, respectively). Bulk samples of the block
copolymers were prepared by solution casting from dichloromethane
(CH.sub.2Cl.sub.2) solution (10 wt % of PS-PLLA) at room
temperature. After quenching from the microphase-separated melt, 10
the thermally treated PS-PLLA samples were subsequently sectioned
by ultramicrotome. A variety of self-assembling nanostructures,
including spherical, hexagonally packed cylindrical, and lamellar
structures, were identified by transmission electron microscopy
(TEM) and small-angle X-ray scattering (SAXS). Similar to the
achiral block copolymer, poly(styrene)-b-poly(D,L-lactide)
(PS-PLA), 11 most of the PS-PLLA samples exhibited typical phase
structures as predicted by the theory based on block copolymer
thermodynamics.
[0061] A nanohelical phase structure was observed for the
PS280PLLA127 (f.sub.PLLA.sup.v=0.35) sample, similar to those shown
FIGS. 2A and 2B. The pitch, projected diameter, and diameter of the
nanohelices were determined, on average, as 43.8,31.9, and 25.3 nm,
respectively. Upon appropriate control of solvent evaporation rate,
well-oriented microdomains consisting of 3D packed PLLA nanohelices
in PS matrix were observed. By examining the phase structure at
different tilting angles in TEM, the nanohelices were found to be
packed in a hexagonal lattice. The occupied volume fraction of PLLA
blocks was calculated to be 31 vol %, which is approximately
consistent with the estimated value from synthetic results
(f.sub.PLLA.sup.v=0.35). Corresponding 2D SAXS experiments using
synchrotron radiations were performed to further identify the
observed morphology. A well-defined scattering pattern having
hexagonal-like diffraction was seen when the X-raybeam was aligned
along the helical axis. One-dimensional SAXS profiles were
extracted from 2D patterns by azimuthal integration as shown in
FIG. 8A. The diffraction peaks were found to occur at q* ratios of
1:4.sup.1/2:7.sup.1/2:9.sup.1/2:13.sup.1/2. These results are
consistent with the hexagonally packed phase structure of which the
nanohelices can be treated as cylinder-like packing. However, even
the reflections in the 2D SAXS diffraction pattern along the
direction of helical axes indicate that the reflections possess
almost identical spacing; the intensity of these reflections
exhibits a 2-fold symmetry as shown in FIG. 8B. Consequently, these
results suggest a pseudo-orthorhombic structure for the packing of
nanohelices (because the ratio of the a-axis and b-axis unit cell
dimensions is nearly equal to) instead of a true hexagonal (it is
probably orthorhombic). We speculate that the 2-fold symmetry in
the intensity for an almost hexagonally packed structure is
attributed to the deformation of nanohelix in uniaxial direction,
in a way similar to the .alpha.-helical conformation of
poly(L-alanine) and the .alpha.-form crystal of PLLA. In the TEM
micrograph, it was found that the nanohelix adopted a left-handed
helical sense, which is consistent with the fact that the PLLA
blocks also possess an L chiral center. As discussed heretofore,
the formation of nanohelical phase structure has never been
reported in PS-PLA block copolymers and PLLA homopolymers before.
We believe that the appearance of the nanohelical structure in the
PS-PLLA may be attributed to the effect of chirality interacting
with the immiscibility of constituted blocks, unlike
noncentrosymmetric phase structures in achiral triblock copolymers
or their blends. Instead of the interplay in the elastic potential
and interfacial energy, the self-assembly of helical curvature is
induced by the formation of specific configurations of PLLA chains
due to the interactions of chiral entities. Similar to the
formation of helical crystals from chiral liquid crystalline
polymers, the chirality strength is strong enough to compete with
self-ordering processes and to stabilize the helical morphology
when the phase structures possess the specific configurations (for
instance, low liquid crystalline order). The studies of detailed
mechanisms are still in process.
EXAMPLE 9
Synthesis of Block Copolymers of
Poly(4-vinylpyridine)-poly(L-lactide), PS280PLLA127
[0062] Block copolymers of Poly(4-vinylpyridine)-poly(L-lactide)
were synthesized using a process similar to those discussed in
previous examples. The volume fraction poly(L-lactide) was
calculated to be about 23% (f.sub.PLLA.sup.v=0.23). As shown in the
TEM micrograph in FIG. 9, a helical microstructure was
observed.
[0063] The foregoing description of the preferred embodiments of
this invention has been presented for purposes of illustration and
description. Obvious modifications or variations are possible in
light of the above teaching. The embodiments were chosen and
described to provide the best illustration of the principles of
this invention and its practical application to thereby enable
those skilled in the art to utilize the invention in various
embodiments and with various modifications as are suited to the
particular use contemplated. All such modifications and variations
are within the scope of the present invention as determined by the
appended claims when interpreted in accordance with the breadth to
which they are fairly, legally, and equitably entitled.
* * * * *